1. Field of the Invention
The present invention relates to an optical receiver.
2. Related Art
An optical receiver converts, into an electrical signal, an optical signal received by a light-receiving element, such as a PIN-type photodiode (PIN-PD) or an avalanche photodiode (APD), and compares this electrical signal with a reference voltage, to thus discriminate an electrical signal. In such an optical receiver, when distortion exists in the waveform of the optical signal, a crosspoint in the waveform of the electrical signal deviates from the reference voltage, thereby degrading receiving sensitivity.
An optical receiver described in Japanese Patent published as JP-H11-346194A alters a threshold value (a reference voltage) used for discrimination even when an overshoot has arisen in the waveform of an optical signal output from an optical transmitter due to the relaxation oscillation; namely, even when the waveform of the optical signal has become distorted, thereby suppressing a reduction in receiving sensitivity. An optical receiver described in Japanese Patent published as JP-2000-134160A suppresses a reduction in the optical sensitivity, by changing a crosspoint in the waveform of a signal input to a discriminator even when a distortion has arisen in the waveform of an optical signal for reasons of a characteristic of blurring of a light-emitting side of a laser light source of an optical transmitter.
However, the optical receiver described in Patent Document 1 installs an AGC circuit as a means for suppressing the reduction in the optical sensitivity, and hence the optical receiver itself becomes large. Moreover, the optical receiver consumes large amounts of power. In the case of the long-distance optical communication, the distortion in the waveform of the optical signal is primarily ascribed not to characteristics of the optical transmitter but to the dispersion in an optical transmission line. The optical receivers described in Patent Documents 1 and 2 detect the intensity of an optical input for suppressing the reduction in the optical sensitivity. However, this is insufficient for suppressing the reduction in the optical sensitivity induced by the distortion of the waveform of the optical signal attributed to the dispersion in the optical transmission line.
Accordingly, the present invention is to provide an optical receiver that is able to suppress a reduction in the optical sensitivity induced by the distortion of the waveform of an optical signal attributed to the dispersion in an optical transmission line, as well as suppressing a reduction in the optical sensitivity induced by a change in the intensity of an optical input without an AGC circuit.